Three-level converters, such as inverters and/or rectifiers, are converters that have three DC poles. In addition to positive and negative DC poles, they have a neutral DC pole. Three-level converter topologies may be used in order to obtain higher DC voltage levels and to reduce a need for output current filtering, for example. Examples of three-level neutral-point-clamped converters are given in T. Brückner, S. Bernet and H. Güldner, “The Active NPC Converter and Its Loss-Balancing Control”, IEEE transactions on industrial electronics, Vol. 52, No. 3, June 2005. In particular, examples of three-level neutral-point-clamped (NPC) converters and active neutral-point-clamped (ANPC) converters are given.
FIG. 1 shows an example of a main circuit of a three-level NPC inverter. The exemplary three-level inverter is a three-phase inverter and thus comprises three switching branches 101, 102, 103 each comprising six diodes and four controllable semiconductor switches. During operation of the inverter the controllable semiconductor switches may be controlled according to a modulation scheme to be used. The exemplary inverter further comprises a positive direct current pole P, a negative direct current pole N and a neutral direct current pole M for connecting the inverter to a DC supply (not shown), such as a DC intermediate circuit. The exemplary inverter further comprises three alternating current poles AC1, AC2, AC3 for supplying a three-phase power system (not shown), for example.
A possible problem related to the above solution is that, especially when the power level of the three-level inverter increases, the commutation loops formed in the three-level inverter may become non-optimal and possibly resulting stray inductances may generate harmful voltage spikes during current commutation between components of the inverter. One particular example of this is when a voltage polarity of the alternating current pole AC1, AC2, AC3 of a switching branch 101, 102, 103 of the three-level inverter is to be changed from positive to negative or from negative to positive and the current is commutated from the upper half of the switching branch to the lower half or from the lower half of the switching branch to the upper half, respectively. In this situation harmful voltage spikes can occur. Similar harmful commutations can occur when the modulated voltage and the output AC current have opposite polarities. One solution to this problem would be to use clamp or snubber circuits to suppress such voltage spikes. However, such circuits may be costly and the operating range may still be limited when compared to a nominal current of the semiconductor switches.